The present invention is related generally to radio frequency (RF) signal filters, and more particularly to an improved single-block ceramic filter that is particularly well adapted for use in radio transmitting and receiving circuitry.
Conventional multi-resonator filters include a plurality of resonators that are typically foreshortened short-circuited quarter-wavelength coaxial or helical transmission lines. The resonators are arranged in a conductive enclosure and may be inductively coupled one to another by apertures in their common walls. Each resonator can be tuned by means of a tuning screw which inserts into a hole extending through the middle of the resonator. Once tuned, the overall response of the filter is primarily determined by the size of the interstage coupling apertures. Since the tuning of such filters can be disturbed by a slight adjustment of the tuning screw, a lock nut is required to keep the tuning screw properly positioned at all times. The use of tuning screws not only renders these filters susceptible to becoming detuned, but also creates additional problems including mechanical locking of the tuning screw and arcing between the tuning screw and the resonator structure. Furthermore, such filters tend to be rather bulky and therefore are relatively unattractive for applications where size is an important factor.
These problems have been solved to some degree by the single-block ceramic filters of the type shown and described in U.S. Pat. Nos. 4,426,631, 4,431,977 and 4,462,098. In particular, the filter described in U.S. Pat. No. 4,431,977 includes a dielectric block having a plurality of circular holes coupled to one another primarily by way of the dielectric between them. Coupling between the circular holes is adjusted by varying the depth of slots or the size and location of additional holes, rendering the tuning of such filters both complicated and difficult. Furthermore, the location of the input and output electrodes near the ground plating creates undesired capacitive loading.